Sulfur-containing fine chemicals such as, for example, methionine, homocysteine, S-adenosylmethionine, glutathione, cysteine, biotin, thiamine, lipoic acid are produced in cells via natural metabolic processes and are used in many branches of industry, including the food, animal feed, cosmetics and pharmaceutical industries. These substances which are collectively referred to “sulfur-containing fine chemicals” include organic acids, both proteinogenic and nonproteinogenic amino acids, vitamins and cofactors. They are most expediently produced on a large scale by means of cultivating bacteria which have been developed in order to produce and secrete large amounts of the substance desired in each case. Organisms which are particularly suitable for this purpose are coryneform bacteria, Gram-positive nonpathogenic bacteria.
It is known that amino acids are produced by fermentation of strains of coryneform bacteria, in particular Corynebacterium glutamicum. Due to the great importance, the production processes are constantly improved. Process improvements can relate to measures regarding technical aspects of the fermentation, such as, for example, stirring and oxygen supply, or to the nutrient media composition such as, for example, sugar concentration during fermentation or to the work-up to give the product, for example by ion exchange chromatography, or to the intrinsic performance properties of the microorganism itself.
A number of mutant strains which produce an assortment of desirable compounds from the group of sulfur-containing fine chemicals have been developed via strain selection. The performance properties of said microorganisms are improved with respect to the production of a particular molecule by applying methods of mutagenesis, selection and mutant selection. However, this is a time-consuming and difficult process. In this way strains are obtained, for example, which are resistant to antimetabolites or inhibitors such as, for example, the methionine analogs α-methylmethionine, ethionine, norleucine, n-acetylnorleucine, S-trifluoromethylhomocysteine, 2-amino-5-heprenoitic acid, selenomethionine, methioninesulfoximine, methoxine, 1-aminocyclopentanecarboxylic acid or which are auxotrophic for metabolites important for regulation and which produce sulfur-containing fine chemicals such as, for example, L-methionine.
Methods of recombinant DNA technology have also been used for some years to improve Corynebacterium strains producing L-amino acids by amplifying individual amino-acid biosynthesis genes and investigating the effect on amino acid production.
WO-A-02/10209 describes a method for the fermentative production of L-methionine using L-methionine-producing coryneform bacteria in which at least the metH gene is overexpressed and also the coding metH sequence from C. glutamicum ATCC 13032.